In a switch mode power supply (SMPS), broadly speaking, a magnetic energy storage device such as a transformer or inductor is used to transfer power from an input side to an output side of the SMPS. A power switch switches power to the primary side of the energy storage device, during which period the current and magnetic field builds up linearly. When the switch is opened the magnetic field (and secondary side current) decreases substantially linearly as power is drawn by the load on the output side.
An SMPS may operate in either a discontinuous conduction mode (DCM) or in continuous conduction mode (CCM) or at the boundary of the two in a critical conduction mode. In this specification we are generally concerned with DCM operating modes in which, when the switching device is turned off, the current on the secondary side of the transformer steadily, but gradually, declines until a point is reached at which substantially zero output current flows and the inductor or transformer begins to ring, entering a so-called oscillatory or idle ring phase. The period of the ringing is determined by the inductance and parasitic capacitance of the circuit.
Referring now to FIG. 1, this shows an example of a SMPS circuit with, merely for example, primary side sensing. The power supply comprises an AC mains input coupled to a bridge rectifier 14 to provide a DC supply to the input side of the power supply. This DC supply is switched across a primary winding 16 of a transformer 18 by means of a power switch 20, in this example an insulated gate bipolar transistor (IGBT). A secondary winding 22 of transformer 18 provides an AC output voltage which is rectified to provide a DC output 24, and an auxiliary winding 26 provides a feedback signal voltage proportionally to the voltage on secondary winding 22. This feedback signal provides an input to a control system 28, powered by the input voltage, e.g., VDD. The control system provides a drive output 30 to the power switching device 20, modulating pulse width and/or pulse frequency to regulate the transfer of power through transformer 18, and hence the voltage of DC output 24. In embodiments the power switch 20 and controller 28 may be combined on a single power integrated circuit. As can be seen, the primary side controlled SMPS of FIG. 1 derives feedback information from the primary side of the transformer, using an auxiliary winding to avoid high voltage signals, the voltage being stepped down by the turns ratio of the transformer. However, alternative techniques for the sensing may be used, e.g. secondary side sensing or other forms of primary side sensing (e.g., sensing a voltage of the primary winding, preferably capacitor coupled so that it can be referenced to the ground of the controller and stepped down using a potential divider, as shown by the inset example circuit of FIG. 1 with a dashed connection to the primary winding 16), and thus the auxiliary winding of FIG. 1 may be omitted.
It may be desirable to set output quantities on an SMPS—voltage, current or power—by the corresponding references in a control loop. These could be fixed or programmable. In certain applications it may further be desirable to change these references while the SMPS is in operation without disturbing the operation of the unit. With an SMPS incorporating an isolation power transformer, the demand for change in the control reference (voltage, current or power) may originate on the secondary (isolated) side of the transformer. If the control loop of such an SMPS utilizes a Primary Side Sensing Controller (PSSC) to control the power (i.e., primary) switch, a separate isolated interface to the PSSC could communicate a demand for change in the control reference(s). Furthermore, it may be desirable to communicate “housekeeping” data (e.g., device temperature, device identification and/or power levels etc.) from the primary (mains side) to the secondary side of the SMPS. Further still, it may be desirable for information to be communicated across the isolation barrier of an SMPS in either direction. The communication could be of analogue or digital format.
Typically, isolation in a communication interface of an SMPS is provided using communication transformer(s) or opto-isolators. Utilizing such isolation devices and the associated components generally results however in an increase in the component count and overall increased cost of the SMPS. This becomes a disadvantage where the design objective is a cost effective and/or programmable output, mains isolated SMPS, for example.
We will describe techniques for providing serial communication using a main power transformer of various types of SMPS (e.g., flyback or forward, using primary side and/or secondary side sensing, single ended and/or using discontinuous mode operation), for example for providing communication between electrically isolated primary and secondary sides at lower cost, reduced component count and/or circuit complexity, increased reliability, etc.
For use in understanding the present invention, the following disclosures are referred to:    U.S. Pat. No. 5,798,913, Tiesinga Jan et al, Philips Corp, date of patent Aug. 25, 1998;    U.S. patent application US2005/0270001 A1, Jitaru Ionel D, Det International Holding Limited, publication date Dec. 8, 2005;    U.S. patent application US2013/0251140 A1, Ransin Johannes G et al, Agere Systems LLC, publication date Sep. 26, 2013;    International patent application, publication number WO2007/005825 A2, Hershbarger Russell, Teridian Semiconductor Corp, publication date Jan. 11, 2007;    DE102012201640, Zelder Thomas et al, PANASONIC CORP, publication date Aug. 8, 2013.